Low carbon baintic steel shows promising potential, especially in critical components
such as frogs and switches. Microstructural analysis of J6 bainitic rail steel was
performed and compared with the microstructure of premium pearlitic rail steel. The
bainitic microstructure revealed a mixture of tempered martensite and ferrite
associated with intralath carbides. Typical pearlitic microstructure with a fine
lamellar aggregate of very soft and ductile ferrite and very hard carbide cementite
was observed. The mechanical properties, plane stress fracture toughness,
, and
the fatigue crack growth behavior of the two steels were evaluated. Test
specimens were machined from railheads of each material using electrical
discharge machining (EDM). Rectangular unnotched and notched specimens
were used for the mechanical properties and fatigue evaluation respectively.
compact tension specimens
were used for the
evaluation according to ASTM E399. The J6 bainitic steel has ultimate strength, yield
strength, and elongation to failure of about 1500 MPa, 1100 MPa, and 13% respectively.
These values are higher than those for pearlitic steel. It was found that the average
for the bainitic rail
steel is 52 MPa
,
while that of the premium pearlitic steel is
41 MPa
.
Fatigue studies showed that the crack speed for the bainitic steel is lower than that
for the pearlitic steel over the entire range of the energy release rate. The
bainitic steel exhibits a higher rate of crack deceleration in the second stage, as
indicated by the lower slope of the fatigue crack propagation kinetics curve in
comparison with the pearlitic steel. This attests to the superior fatigue damage
tolerance of the bainitic rail steel and provides evidence to support the superior
rolling fatigue damage tolerance of the bainitic rail steel reported in the
literature.
